3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Closure estimations from underground observations and their comparison to closure from elastic numerical modelling(2017) Rakumakoe, ObakengThe gold reserves in South Africa have been mined for decades, depleting all the easily accessible reserves. In pursuing the deeper reserves South African mining industry has for many years led the development of mining and particularly rock engineering. Various design criteria and tools have been developed and used by South African rock engineers in different mining environments. It must also be understood that these criteria were developed decades ago in different mining environments compared to where mining is currently taking place. In using these design criteria one needs to look at the relevance of such criteria and question if they are still applicable or if new criteria are required. Scheepers et.al, (2012) reviewed the design criteria used in designing ultra-deep narrow reef stopes in the West Wits and identified that there was no clear correlation between the design criteria used and the seismicity which is the highest FOG risk in ultra-deep mines. They then decided to use modelled elastic closure as design parameter which can be correlated to seismicity. This report details an investigation into the correlation of the modelled elastic closure to the estimated closure from underground and how modelled closure can be adjusted to better reflect the anticipated closure underground. The investigation was conducted using underground observations and stoping width estimations using installed timber support and numerical modelling results (MAP3D). Before correlating the modelled closure and the estimated closure, it was critical to understand the basis of the work done by (Scheepers et.al, 2012) in correlating the modelled closure to seismic hazard. McGarr, (1976) introduced the concept of correlating seismic energy to volume changes in stope. However this correlation was on the basis that the closure in the stope is only as a result of seismic failure. This was the basis of work done by (Scheepers et.al, 2012) in correlating volume change due to seismicity (seismic potency) to modelled closure. It must be understood that (Scheepers et.al, 2012) aim was not for the modelled closure to reflect underground closure, however was to give an indication of the anticipated seismic activity relative to closure. This report further looks at what would the underground closure be relative to the modelled closure which has been used as a design parameter against seismicity. This report showed no correlation between the 0.27m modelled closure determined by Scheepers et.al, (2012) for Mponeng mine to the estimated closure. Through (Scheepers et.al, 2012) work, it was also shown that the correlation of potency to modelled closure was only in the first 10000m2 of mining a new raise line. Seismic potency is highly dependent on the seismic moment of a seismic event and the larger the event, the larger the seismic potency without any consideration to the mining layout. The elastic modelled closure was found to be on average only 55.3% of the estimated closure. The MAP3D model only considered the elastic properties of the rock and did not take into account any discontinuities or non-homogeneity in the rock mass, hence the large difference to the measured closure. It is important to note that seismic potency and elastic closure modelled do not take into account critical factors that contribute to both rock mass deformation and seismicity in deep mines. More work is required to gain a better understanding on the correlation of rock mass deformation in ultra-deep mines to seismicity. Of importance from the research is to acknowledge that the use of modelled elastic closure should always be supported with a good understanding of the actual rock mass behaviour. The elastic properties used in numerical modelling programs could be varied in such a way that the elastic modelling results can closely depict the actual rock mass behaviour in terms of closure. Accurate estimation of closure would be useful in the design of support systems and mining layouts in ensuring the stability of excavations for the required periods. Closure can be estimated by conducting underground measurements and calculated by running numerical modelling programs. Better correlations between the two results would be possible once the elastic properties used in a model are varied until the results obtained from the model are similar to the underground measurements. The inclusion of the backfill material into the elastic model has significant influence on the resultant closure. This was shown by varying the stoping width used in the model. In a pure elastic model without backfill the stoping width has no influence on the resultant modelled closure as it is evident in the elastic closure formula by (Malan, 2003) which does not take into account stoping width. Varying the poison’s ration has very little influence of the modelled closure while the adjustments to the young’s modulus has a significant influence to the modelled closure.Item A study of failure in the rock surrounding underground excavations(1962) Cook, Neville G.W.Violent failure of the rock surrounding under ground excavations forms a major hazard and obstacle in deep-level mining. (Abbreviation abstract)Item Prediction of the initiation and orientation of the extension fractures ahead of and around faces and walls of mechanically driven excavations and their effect of stability(2017) Mokgohloa, Matthews ChueneBoring of shafts and tunnels in hard rock mines is more prevalent in recent years in South Africa. This normally takes place under substantial stress conditions, where fracturing of rock occurs around the boundaries and ahead of advancing faces of excavations. Fracturing can have a significant impact on boring activities, in some instances causing sidewall spalling which can be extensive, with machine grippers unable to reach the sidewalls. In brittle rock, these fractures are commonly extensional in nature. This research has been undertaken to predict the initiation of extension fractures and their orientations ahead of machine driven tunnels. Furthermore, it will help to assess the stability of the excavations, by evaluating the potential for slab/plate failures. This was based on the typical in situ stress fields for underground deep level mines. The numerical analyses involved the generation of different plots: Principal stress contour plots, depicting stress distributions around and ahead of tunnel excavation, using cutting planes; Isosurfaces, showing zones of extension or potential extents of fracturing, applying the extension strain criterion; and Trajectory ribbons, to demonstrate the orientations of fractures. Based on the results of the stress analyses, potential slab or plate formation was determined. It was noted that the fracture zone is a function of a tunnel size. For instance, a four-metre diameter tunnel is less likely to give boring problems than an eight-metre diameter tunnel. The failure of the tunnels was predicted by employing slab analysis methods. An eight-metre diameter tunnel had slenderness ratio as low as 22.3 as compared with a four-metre diameter tunnel with a slenderness ratio of 27. Looking at buckling stress versus slenderness ratio, this translates to buckling stress values of above 100 MPa for an eight-metre tunnel and to values just below 50 MPa for a four-metre tunnel. The outcome of the research gives a clear indication that boring activities could be undertaken under severe conditions. This could be detrimental to the cutter head, since large slabs and blocks could be encountered during boring. The results of this research can be beneficial in the evaluation of boring conditions prior to and during boring activities.Item Identification of inelastic deformation mechanisms around deep level mining stopes and their application to improvements of mining techniques.(2014-02-26) Kuijpers, J.S.Mining induced fracturing and associated deformations can commonly be observed around deep gold mining excavations. As the rockmass behaviour and the stability of the excavations are directly influenced by these processes, a proper understanding of this influence would certainly improve current mining practices with respect to blasting, rock breaking, support design and mining lay-outs. The main subject of this thesis is the physics of failure and post failure behaviour of rock and similar materials. Failure is denned here as a state at which the material has been subjected to fracture and/or damage processes. The applicability of commonly used constitutive models in representing such failure and post failure processes has been investigated mainly by means of numerical simulations. Mechanisms which control fundamental fracture and damage processes have been analysed by comparing the results from relevant laboratory experiments with numerical models. Linear elastic fracture mechanics has been applied to explain and simulate the formation of large scale extension fractures which form in response to excessive tensile stresses. Using the flaw concept it is demonstrated that these fractures not only initiate and propagate from the surface of an opening in compressed rock, but that so called secondary fracturing can be initiated from within the solid rock as well. The effect of geological discontinuities such as bedding planes, faults and joints on the formation of (extension) fractures has also been investigated and it has been shown how the presence of such discontinuities can cause the formation o f additional fractures. Micro mechanical models have been, used to investigate the interaction and coalescence processes of micro fractures. It was found that the formation of large scale extension fracturing can be explained from such processes, but so called shear fractures could not directly be reproduced, although such a possibility has been claimed by previous researchers. The formation of shear fractures is of particular- interest as violent failure of rock, which is subjected to compressive stresses only, is often associated with such fractures. In an all compressive stress environment, only shear deformations would allow for the relief of excess stress and thus energy. The formation of shear fractures is associated with complex mechanisms and shear fractures can therefore not directly be represented by tingle cracks. In contrast to the propagation of tensile fractures, which can readily be explained by traditional fracture mechanics in terms of stress concentrations around the crack tip, the propagation of shear fractures requires a different explanation. In this thesis an attempt has nevertheless been made to reproduce shear fractures by direct application of fracture mechanics. This his been done by representing a shear fracture as a single crack and by assuming fracture growth criteria which are either based on critical excess shear stresses, or on a maximum energy release. Both criteria are completely empirical and require a value for the critical shear resistance in the same way as a critical tensile resistance is required to represent the formation of tensile fracture; , The determination of a critical tensile resistance ( Kk ) is relatively straight forward, as the formation of tensile fractures from a pre-existing flaw can be reproduced and observed in standard laboratory tests. The determination of a critical shear resistance is, however, not a common practice, as the formation of a shear fracture from a pre-existing flaw is very infrequently observed. The application of shear fracture growth criteria nevertheless resulted in plausible fracture patterns, which suggests that such criteria are realistic. It is argued here however that the formation of shear fractures cannot be associated with primary fracture growth, but rather with the localisation of failure and damage in an area which is subjected to plastic deformation. The application of fracture mechanics is therefore not correct from a fundamental point of view as these processes are not represented. For this reason plasticity theory has also been applied in order to simulate failure in general, and shear failure localisation in particular. It was in principle possible to reproduce the shear fractures with the use of this theory, but numerical restraints affected the results to such an extent that most of the simulations were not realistic. Plasticity theory can also be extended to include brittle behaviour by the use of so called strain softening models. The physical processes which lead to brittle failure are however not directly represented by such models and they may therefore not result in realistic failure patterns. It was in fact found that strain softening models could only produce realistic results if localisation of failure could be prevented. The effect of numerical restraints becomes even more obvious with a strain softening model in the case of failure localisation. While the plasticity models appear inappropriate in representing brittle failure, they demonstrated that plastic deformations can be associated with stress changes which may lead to subsequent brittle fracturing. Although only indirect attempts have been made to reproduce this effect, as appropriate numerical tools are not available, it is clear that many observations of extension fracturing could be explained by plastic deformations preceding the brittle fracturing processes. Many rocks are classified as brittle, but plastic deformation processes often occur during the damage processes as well. The sliding crack for instance, which is thought to represent many micro mechanical deformation processes in rock, directly induces plastic deformations when activated. A pure brittle rock, which may be defined as a rock in which absolutely no plastic deformation processes take place, may therefore only be of academic interest as it is inconceivable that such a rock materiel exists. Only in such an academic case would (linear) elastic fracture mechanics be directly applicable. As plastic deformation processes do play a role in real rock materials it is important to investigate their influence on subsequent brittle failure processes. The elastic stress distribution, which is often used to explain the onset of brittle fracturing, may be misleading as plastic deformations can substantially affect the stress distribution . -recediny fracture initiation. In an attempt to combine both plastic and brittle failure, use has been made of tessellation models, which in effect define potential fracture paths in a random mesh. The advantage of these models is that various failure criteria, with or without strain softening potential, can be used without the numerical restraints which are normally associated with the conventional continuum models. The results of these models are also not free from numerical artefacts, but they appear to be more realistic in general. One o f the m;ij, r conclusions based on these results is that shear failure does not occur in a localised fashion, but is associated with the uniform distribution and extension of damage. Shear failure, which can be related directly to plastic failure, can however induce brittle, tensile, failure due to stress redistribution. While the theories of fracture mechanics and plasticity are well established, their application to rock mechanical problems often leads to unrealistic results. Commonly observed firacture patterns in rock, loaded in compression, are most often not properly reproduced by numerical models for a combination of reasons. Either a model concentrates on the discrete fracturing processes, in which case the plastic deformation processes are ignored, or plasticity is represented, but brittle failure is pooxiy catered for. While theoretically a combination of these models might lead to better representations and simulations, numerical problems do affect all models to a certain extent and a practical solution is not immediately available. The results of numerical models can therefore only be analysed with caution and the underlying assumptions and numerical problems associated with a particular technique need to be appreciated before such results can be interpreted with any sense. Many of the problems are identified here and this may assist researchers in the interpretation of results from numerical simulations. Laboratory experiments, which have been chosen for analyses, involve specimens which have been subjected to compressive stresses and which contain openings from which failure and fracturing is initiated. Such specimens are less subjective to boundary influences and are far more representative of conditions around mining excavations than typical uni- and tri-axial tests. The uniform stress conditions in these latter tests allow boundary effects to dominate the stress concentrations, and thus failure initiation, in the specimens. The large stress gradients, which can be expected to occur around underground excavations, are not reproduced in such specimens. As a consequence failure is not u atained within a particular area, but spreads throughout the complete specimen in the uni- and tri-axial tests. Specimens containing openings are therefore far more likely to reproduce the fracture patterns which can be observed around deep level mining excavations. Numerical simulations of brittle, tensile fracturing around mining excavations resulted in consistent fracture patterns. Fracture patterns could however be strongly influenced by the presence of geological (pre-existing) discontinuities such as bedding planes. Although tensile stresses are often assumed to be absent around deej:Item Use of joint trace data to evaluate stability of mining excavations, and validation against underground observations(2012-09-20) Nezomba, EdgarMining is a sensitive business that yields high returns and at the same time is associated with high risk of injuries/fatalities and potential losses of revenue. There is increasing intolerance for injuries and fatalities by governments and the other stakeholders involved in the mining business often resulting in mine closures and revenue loss. Chief among the mining risks is the occurrence of rockfalls where people work and access. The rockfalls are bound mainly by joints that intersect the rockmass thereby forming rock blocks that may fall once an excavation has been created. There are many methods that have been used over time to predict the occurrence of rockfalls. More recently probabilistic methods have gained more ground over deterministic methods. The properties of the joints that are identifiable on exposed excavations are the main inputs used in simulating rockfalls. To date there has been little work that has been done to compare predicted rockfalls to actual rockfalls. This dissertation presents a practical method for collecting rockfall and joint data in the stope hangiwall at two mines in the Bushveld Complex. The joint data has been used in simulating rockfalls using JBlock (a probabilistic keyblock stability programme). A comparison between simulated rockfalls and mapped rockfalls has been presented. Based on this comparison, a number of iterations were done to calibrate the JBlock results until near realistic rockfalls were achieved. Three case studies have been conducted to investigate the effectiveness of different stope support systems in reducing rockfall. The potential losses and injury risk associated with the different support systems have been quantified for all the individual rockfalls. In general the rockfall frequency is directly proportional to the risks associated with the rockfalls. Through this research it has been demonstrated that it is possible to use joint data found on excavation surfaces to statistically predict the occurrence of potential rockfalls in similar ground conditions. The optimum support system that has minimum injury and cost risk can also be selected from a comparison of a number of support systems. Armed with this information, rock engineers can now make strategic decisions versus the existing common tactical approach.